With such an apparatus so-called eddy current separation can be carried out. The material is conveyed over the poles of an alternating magnetic field generator, for example on a conveyor or in free fall. By this means eddy currents are induced in the electrically conductive components of the mixture which build up their own magnetic fields opposed to the generating field and thereby accelerate these components, through electromagnetic forces, relative to the other components of the mixture. Non-ferromagnetic materials of good electrical conductivity, such as aluminium and copper, can be separated from non-ferrous solid mixtures and non-ferrous metal/non-metal solid mixtures, such as car shredder scrap or electronic scrap metal by means of eddy current separation. Should there be ferromagnetic fractions in this material a magnetic separator can be arranged before the eddy current separator to first remove ferromagnetic fractions. In addition other sorting and classifying stages are advantageously arranged before the eddy current separation because pre-enrichment and fractionation of the charged solid mixture to the greatest possible extent has a good effect on the success of separation.
In a separating apparatus known from DE-OS 34 16 504, in order to separate the ferromagnetic fraction a solid mixture is first transported by means of a conveyor belt beneath a magnetic separator and thereafter fed from the conveyor belt to the outside of a slowly rotating drum to separate out the non-ferrous metals. Arranged concentrically in the interior of the drum is a rapidly rotating rotor fitted with permanent magnets. The permanent magnets extend uniformly parallel to the rotor axis and are arranged at a large distance from one another so that the magnetic field forming between the poles of the permanent magnets acts as far as possible outside of the drum. In comparison to other eddy current separating processes this known apparatus is said to enable higher throughput to be obtained with thicker layers of the solid mixture because the separating forces of the alternating magnetic field already act on the solid mixture at the time when the forces of gravity have no or only a little effect.
However, with this known apparatus there is mutual interference if the material particles go beyond the radius of the drum into their trajectory parabola. On the one hand conductive particles to be diverted are retarded by the non-conductive particles and on the other hand non-conductive particles are accelerated undesirably owing to the contact with the conductive non-ferrous metal particles. As a result it is not possible to avoid misplaced materials in both the products, i.e. electrically non-conductive particles discharged into the collecting region of the non-ferrous metal particles and vice versa. Apart from this, accommodating the magnetic rotor in the space in the drum presents considerable problems; these involve both constructional and manufacturing difficulties. Thus the magnetic rotor must be mounted in the restricted space within the preferably rotatable drum, the diameter of which cannot be increased at will, and the mounting becomes still more complicated if the magnetic rotor is to be adjustable, for example concentrically around a radius or on a curve at different radial distances from the axis of rotation of the drum.
Furthermore the drum can only be manufactured or machined with difficulty and requires extremely accurate finishing to obtain desired thin, uniform wall thicknesses of the drum with high mechanical stability so that as far as is possible no magnetic force is lost. For example there must not be differences in the hardness of the material in the surface of the drum, i.e. no softer or harder areas must arise as a result of which the very small air gap between the magnetic rotor and the drum might be locally reduced so that serious damage resulting from frictional contact between the magnetic rotor and the drum could occur.